1. Field of the Invention
The present invention relates to an optical disk file device, and more particularly to a drive system used in such a device for controlling the position of a read/write head with respect to the optical disk and for driving the read/write head to track the optical disk.
2. Description of the Prior Art
Optical disk file devices have a read/write head for writing and reading information in and out of an optical disk by illuminating a light spot on a desired track on the optical disk, the track being more or less eccentric with respect to the disk. To align the light spot with the desired track, the read/write head is subjected to tracking operation which is carried out by drive means including an objective lens in the read/write head, which is movable radially across the optical disk. This drive system has been disadvantageous in that the movement of the objective lens results in a displaced optical axis of the optical system in the read/write head, and hence any tolerable degree of eccentricity of the tracks on the optical disk is reduced. The above difficulty would be eliminated by moving the overall head optical system, rather than the objective lens only, radially across the optical disk. This proposal however would still suffer from a drawback in that the weight of the entire movable mechanism would be large, and the tracking motor would be large in size. There has been adopted a system that has been studied at an early stage which includes a tracking motor doubling as an accessing motor for moving the read/write head to a position near a designated track. The motor employed comprises a voice-coil linear motor used in magnetic disk drives. This type of linear motor has stable controllability, but needs a large-size magnetic circuit. Such linear motors can be rendered smaller in size by having two-phase coils energizable with phase switching. The phase switching drive system includes a circuit for suppressing ripples in the motor thrust, but is unable to restrain instantaneous thrust variations occurring at the time of phase switching. When tracking happens to be effected simultaneously with phase switching, proper tracking with the accuracy on the order of submicrons is impossible to achieve due to such thrust variations. The phase switching drive system is costly to construct as the circuit for suppressing thrust ripples is generally complex in arrangement.